The IEEE 802.16-2005 specification (IEEE Standard for Local and Metropolitan Area Networks—Part 16: Air Interface for Fixed Broadband Wireless Access Systems) specifies the air interface for a wireless communication system widely known as WiMAX. In particular, the IEEE 802.16-2005 specification defines frame structures for Time-Division Duplex (TDD) and Frequency-Division Duplex (FDD) deployments of the Orthogonal Frequency Division Multiple Access (OFDMA) physical (PHY) layer. Current developments of the standard as deployed for Mobile WiMAX are focused on TDD systems, where the carrier is divided in time between the downlink (DL) and the uplink (UL) directions.
In one possible deployment of the system as used by Mobile WiMAX, the carrier can have a bandwidth of either 5 or 10 MHz. In either case, the frame duration is five milliseconds, and is apportioned into downlink and uplink subframes that are separated by transmit-receive transition gaps (TTGs) and receive-transmit transition gaps (RTGs) to allow for switching between transmit and receive directions. The uplink and downlink subframes are in turn divided into subchannels, such that a subchannel comprises a particular number of OFDMA subcarriers (frequency resources) over a “slot” comprising two or three OFDMA symbol times (time-domain resources). The exact structure of the uplink and downlink subchannels depends on whether the time-frequency resources at issue are in a Partial-Use-of-Subcarriers (PUSC) zone, a Full-Usage of Subcarriers (FUSC) zone, or an Adaptive Modulation and Coding (AMC) zone. The IEEE 802.16-2005 standard also specifies the allocation of time-frequency elements for pilot signals, again according to the zone type.
Several transmission formats are defined for Mobile WiMAX. The selection of a particular transmission format is based on the signal processing capabilities of the base station and the mobile station, as well as the number of antennas deployed at each radio. Among these transmission formats are several multi-antenna configurations collectively known as Multi-Input Multi-Output (MIMO) modes. These MIMO modes include a space-time coding mode, using Alamouti coding, and a spatial multiplexing mode.
Mobile WiMAX allows users to effectively choose the best MIMO mode on the downlink, between Alamouti coding and spatial multiplexing, a technique called dynamic switching. The mobile station sends a channel quality indicator (CQI) report with an estimate of the signal-to-interference ratio (SINR) on the downlink channel. In addition, the mobile station may indicate a MIMO mode to use on the downlink, using an associated fast feedback channel. The base station allocates time-frequency resources on the downlink, using this information, and signals the resource allocations in a DL-MAP message transmitted near the beginning of each downlink frame.
It is well known that the rank of the transmission channel plays an important role in the number of spatially multiplexed streams that the channel can support. For an M-by-N MIMO scheme (M transmit antennas and N receive antennas), the rank can vary between 1 and the lesser of M and N. Attempting a transmission of more streams than the rank will cause degraded performance. Thus, for example, if an M-by-N MIMO channel has rank P, then the supported rate of the transmission is highest when P streams are transmitted. Rank adaptation is used in various systems, such as the Long-Term Evolution (LTE) system developed by the 3rd-Generation Partnership Project (3GPP), as a capacity enhancing measure.
WiMAX supports a 2×2 Alamouti space-time coding scheme, which represents a single-stream MIMO transmission. In addition, WiMAX supports a 2×2 spatial multiplexing mode, which represents a two-stream MIMO transmission. Thus, the dynamic switching as used in Mobile WiMAX can be viewed as emulating a form of rank adaptation. With dynamic switching, different users in the same zone may be using different MIMO transmission schemes at any given time.